Symptom & fix Monitor it

Worse gas mileage than usual

A sudden drop in MPG often traces to sensors, tires, or a fuel-mixture problem rather than the engine itself.

What this usually means

A gradual MPG decline can just be winter fuel, short trips, or aging tires. But a noticeable, sudden drop usually means the engine is burning more fuel than it should — commonly because a sensor is feeding the computer bad data and the mixture has gone rich, or because something is making the engine work harder than normal. It’s worth tracking down because the same faults that waste fuel often raise emissions and can trip the check-engine light.

Most likely causes

high
Failing oxygen (O₂) sensorA lazy O₂ sensor makes the computer over-fuel the engine, directly hurting mileage.
medium
Dirty or failing mass airflow sensorA bad MAF reading skews the air-fuel ratio and wastes fuel.
medium
Under-inflated tires or dragging brakesExtra rolling resistance forces the engine to work harder — a cheap, often-overlooked cause.
medium
Faulty thermostat (engine running cold)An engine stuck below operating temperature stays in a fuel-rich warm-up mode and drinks gas.
low
Clogged air filter or worn spark plugsRestricted air or weak spark hurts combustion efficiency and economy.

How urgent is it?

Monitor it. Poor fuel economy is costing you money every tank, and the underlying sensor or mixture fault often gets worse. Worth diagnosing rather than ignoring — and if the check-engine light starts flashing, stop driving and get it checked.

Redline can’t determine from a symptom or code alone whether your car is safe to drive.

Typical fix & cost

Start with the cheap checks: tire pressure, air filter, and any pending codes. From there it’s usually an O₂ or MAF sensor, a thermostat, or a tune-up. Confirming which sensor is actually lying avoids replacing the wrong one.

Typical range: $40–$500

Tires/filter are cheap; sensors and a thermostat are moderate.

The price depends on which cause it turns out to be, so confirm the cause before paying. Diagnose this for my car →

Which cause is it on your car?

Start a case with what you’re noticing. Redline weighs these causes against your car’s evidence and names the check that would settle it.

Diagnose my car

Related OBD-II codes

If your car has stored a trouble code, these often come with this symptom:

P0171 P0172 P0128 P0420

Related symptoms

Check engine light is on Engine idles rough or shakes at a stop Engine temperature runs hot or overheats

General guidance, not a substitute for a hands-on inspection. Cost ranges are broad reference ranges, not quotes. For safety-related issues, have the car inspected before driving.

Start a case

Seeing this on your car?

Every problem becomes a Case. Tell Redline what the car is doing: it shows what the evidence supports, what’s still missing, and what to check next, before you approve a repair.

Case updatedv2

Braking grind · 2016 Toyota Camry

“Inner pad ~1 mm. Outer pad 3 mm.”

Sticking caliper / slidePossible → Supported

Worn rotor onlyPossible → Less likely

Next: check the slide pins move freely before buying parts.

Example